The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics and toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,459,426 (Inwood et al.) discloses a liquid-phase transalkylation process, which is used in conjunction with an olefin alkylation process, that converts a poly-alkylaromatic mixture into additional mono-alkylaromatic compounds, such as ethylbenzene. This disclosure teaches that only trace amounts of xylenes, which are highly undesirable for such a process, are produced in amounts less than 0.2 wt-percent.
U.S. Pat. No. 5,004,855 (Tada et al.) discloses a process for ethylbenzene destruction within a C8 alkylaromatic mixture. U.S. Pat. No. 6,342,649 B1 (Winters et al.) also discloses a method of removing ethylbenzene from a C8 alkylaromatic mixture. Both of these disclosures teach conversion of the ethylbenzene component to benzene by irreversible de-ethylation.
Other types of transalkylation processes have been disclosed. U.S. Pat. No. 5,847,256 (Ichioka et al.) discloses a process for producing xylene from a feedstock containing C9 alkylaromatics with ethyl-groups over a catalyst containing a zeolite component that is preferably mordenite and with a metal component that is preferably rhenium. U.S. Pat. No. 5,942,651 (Beech, Jr. et al.) discloses a flowscheme for a gas-phase transalkylation process in the presence of two zeolite containing catalysts to produce xylenes and benzene. The first catalyst contains a hydrogenation metal component and a zeolite component from the group including MCM-22, PSH-3, SSZ-25, ZSM-12, and zeolite beta. The second catalyst contains ZSM-5, and is used to reduce the level of saturate co-boilers necessary for a high-purity benzene product. U.S. Pat. No. 5,952,536 (Nacamuli et al.) discloses a gas-phase transalkylation process using a catalyst comprising a zeolite from the group including SSZ-26, Al-SSZ-33, CIT-1, SSZ-35, and SSZ-44. The catalyst also comprises a mild hydrogenation metal such as nickel or palladium, and is used to convert aromatics with at least one alkyl group including benzene.
Economical processes in the field of integrated aromatics complexes are continually sought having exceptionally high selectivity for xylenes from other aromatic intermediates.